1. Field of the Invention
This invention relates to stable colorants of high extinction coefficient and high solubility or dispersibility for an electrophoretic, magnetophoretic or electromagnetophoretic display. More particularly, it relates to stable colorants for a microcup-based electrophoretic, magnetophoretic or electromagnetophoretic display the cells of which are filled with charged and/or magnetic particles dispersed in a halogenated, preferably a fluorinated, solvent. The use of the stable colorants allows the display to be of superior contrast ratio and longevity, and suitable for high-quality imagery applications.
2. Brief Description of Related Art
The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing the migration of charged pigment particles in a solvent, preferably a colored dielectric solvent. This type of display was first proposed in 1969. An EPD typically comprises a pair of opposed, spaced-apart plate-like electrodes, with spacers predetermining a certain distance between the electrodes. At least one of the electrodes, typically on the viewing side, is transparent. For the passive type of EPDs, row and column electrodes on the top (the viewing side) and bottom plates respectively, are needed to drive the displays. In contrast, an array of thin film transistors (TFTs) on the bottom plate and a common, non-patterned transparent conductor plate on the top viewing substrate are required for the active type EPDs.
An electrophoretic fluid composed of a colored dielectric solvent with charged pigment particles dispersed therein is enclosed between the two electrodes. When a voltage difference is imposed between the two electrodes, the pigment particles migrate by attraction to the plate of polarity opposite that of the pigment particles. Thus, the color showing at the transparent plate, determined by selectively charging the plates, can be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite plate, thereby reversing the color. Intermediate color density (or shades of gray) due to intermediate pigment density at the transparent plate may be obtained by controlling the plate charge through a range of voltages or pulsing time.
EPDs of different pixel or cell structures have been reported previously, for example, the partition-type EPD (M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., Vol. ED 26, No. 8, pp. 1148-1152 (1979)) and the microencapsulated EPD (U.S. Pat. Nos. 5,961,804 and 5,930,026).
An improved EPD technology was recently disclosed in co-pending applications, U.S. Pat. No. 6,930,818 (corresponding to WO 01/67170), U.S. Pat. No. 6,672,921, (corresponding to WO 02/01281) and U.S. Pat. No. 6,933,098, (corresponding to WO 02/65215), all of which are incorporated herein by reference. The improved EPD comprises isolated cells formed from microcups and filled with charged particles dispersed in a dielectric solvent. The filled cells are individually sealed with a polymeric sealing layer, preferably formed from a composition comprising a material selected from a group consisting of thermoplastics, thermosets and precursors thereof.
Other types of displays, namely magnetophoretic displays (MPDs) and electromagnetophoretic displays (EMPDs), are disclosed in U.S. Pat. Nos. 6,927,892 and 6,914,713, filed on Apr. 23, 2002, the contents of both are incorporated herein by reference in their entirety.
The magnetophoretic display generally comprises display cells sandwiched between two layers of substrate and filled with a magnetophoretic dispersion wherein the pigment particles are magnetic but not charged. The display is driven by a magnetic field. At least the substrate layer on the viewing side is transparent.
In the electromagnetophoretic display, the display cells sandwiched between two substrate layers are filled with an electromagnetophoretic fluid wherein the pigment particles are both charged and magnetic. One of the substrate layers, preferably on the non-viewing side, is coated with a conductive layer facing the filled display cells. The display is driven by a combination of electric and magnetic fields. The substrate layer on the viewing side is transparent.
For all types of displays, the dispersion contained within the display cells is undoubtedly one of the most crucial parts of the device. The dispersion, as stated earlier, usually is composed of pigment particles dispersed in a colored dielectric solvent or solvent mixture. The composition of the dispersion determines, to a large extent, the longevity, contrast ratio, switching rate, response waveform and bistability of the device. In an ideal dispersion, the dispersed pigment particles remain separate and do not aggregate or flocculate under all operating conditions. Furthermore, all components in the dispersion must be chemically and electrochemically stable and compatible not only with each other but also with the other materials present in a display, such as the electrodes and sealing and substrate materials.
The dispersing medium may be colored by dissolving or dispersing a dye or colorant in the dielectric solvent or solvent mixture.
Halogenated solvents of high specific gravity have been widely used in EPD applications, particularly in those involving an inorganic pigment, such as TiO2, as the charged whitening or coloring particles. The halogenated solvents of high specific gravity are very useful in reducing the rate of sedimentation of the pigment particles in the solvent. Fluorinated solvents are among the most preferred because they are chemically stable and environmentally friendly.
However, most dyes or pigments are not soluble in fluorinated solvents, particularly not in high boiling-point perfluorinated solvents. For example, phthalocyanines are highly desirable colorants due to their high extinction coefficients, narrow absorption bands and chemical stability; but they are normally insoluble in most solvents, and are particularly insoluble in fluorinated solvents. Therefore, displays based on fluorinated dielectric solvents colored by this type of dyes typically show poor shelf-life stability, contrast ratio and switching performance.
Certain soluble fluorinated copper phthalocyanine dyes are disclosed in U.S. Pat. No. 3,281,426 (1966). The process for the preparation of these dyes involves heating a mixture of an aromatic starting compound and a perfluoroalkyliodide at a temperature in the range of from 200° C. to 350° C. The reaction is performed in an autoclave or a pressure ampoule due to the pressure developed. This synthesis involves complicated reaction conditions (e.g., high pressure and temperature) and long reaction time and has a low yield. Other phthalocyanine derivatives (U.S. Pat. Nos. 6,043,355 and 5,932,721) show improved solubility in various organic solvents or even in water, but not in highly fluorinated solvents.
Thus, there is a need for stable dyes or colorants that exhibit high solubility or dispersibility in halogenated, particularly fluorinated, dielectric solvents for use in display applications. The dyes or colorants should also have a high extinction coefficient, narrow adsorption bands and chemical or electrochemical stability, and can be manufactured in high yields at low cost.